If a TV show nowadays doesn't have at least one big twist now and then, chances are it doesn't remain on the air for long. But only a handful of TV series have the courage to completely transform themselves — to show something so crazy, it irrevocably redefines the show from that point on. We've been lucky to have several shows, especially recently, with these game-changing episodes — here are a dozen.

1) Game Of Thrones, "The Rains of Castamere"

Game of Thrones is constantly changing its status quo, but none have come close to the "Red Wedding" in the penultimate episode of season 3. Not only do we lose two beloved protagonists in Catelyn and her son Robb, but we also lose a major portion of what we thought the show was about — a young man battling to avenge his father and free his country. Which the Starks dead and/or scattered, the Stark/Lannister conflict — central to the entirety of the show up to that point— is effectively over, and the Lannisters have won. An entirely new story is taking its place.

Lost is a show that was built on shocks, which it provided all the way through its season six finale. But no shock was more shocking that the season 3 finale, where Jack yells to Kate "We have to go back!" and suddenly we realized we've been in a flash-forwards, not the usual flashbacks. Now instead of trying to get off the island, some of the characters are getting back on it, a completely different journey — both physically and thematically. Moreover, this episode showed that Lost was going to play around with time in ways that would greatly affect the later seasons in virtually all aspects.

3) Buffy the Vampire Slayer, "Buffy Vs. Dracula"

Whether you love Buffy's little sister Dawn or hate her — and chances are you probably hate her — you can't deny that she uprooted Buffy, both the character and the show. Sure, as a mystical key hidden in human form, she provided season 5 with its plot, and suddenly gave Buffy a new, pre-built relationship. But not only did Dawn shift the key dynamics of the show, she turned Buffy into a surrogate caregiver, which came to define her on the show. Admittedly, Buffy wasn't particularly good at at most of the time, but Dawn's arrival gave Buffy a set of stakes (no pun intended) that she'd never had before, and thus her mistakes (still not a pun) had a cost they'd also never had before.

4) Battlestar Galactica, "Crossroads"

It was the question everyone wanted to know: Who are the Final Five? It turns out even the Final Five wanted to know who the Final Five were, as they were sleeper agents Tigh, Anders, Tyrol, and Tory (the fifth being Tigh's wife Ellen, who had already been executed). The reveal that these four beloved characters had secretly Cylons all along devastated both them and the viewers, and it set the scene for Battlestar Galactica's final act, an act that was also precipitated by the miraculous return of Starbuck, who claimed to have found Earth. The show suddenly had a new purpose, a new focus on the miraculous that would heavily influence the final episodes, and, thanks to the Final Five, the realization that literally anything could happen in these final episodes.

5) Adventure Time, "Simon & Marcy"

We've praised Adventure Time's "Simon & Marcy" episode time and time again, and there's a reason: It's incredible. Not only does it change Adventure Time moving forward, it changes all the earlier episodes too, because we find out that Finn and Jake's primary antagonist, the Ice King, tragically gave his sanity away to protect a young Marcy in the aftermath of the Great Mushroom War. It revealed the previous evil/goofy Ice King as a tragic character, and changed the way the thought about the show's primary conflict. It was also the beginning of a maturity of storytelling that Adventure Time has previously hinted at, and one that continues to this day, in storylines like Jake's fatherhood, Finn's subtle journey through puberty, Princess Bubblegum's increasingly sinister ways, Finn's horrible father, and more.

6) Agents of SHIELD, "Turn Turn Turn"

You could easily say that "Turn, Turn, Turn" is a game-changer as it's when Agents of SHIELD finally got good. But that's because it brought so much to the series it badly needed — a clear, definable, powerful antagonist; a reason to care about the characters; actual danger; and, most of all, a great twist. Revealing that Ward had been a secret Hydra agent for the entire season — a fact we suddenly knew but the other Agents did not — raised the stakes for the team dramatically, just as the reveal of Hydra (tied to the Captain America: The Winter Soldier movie) gave the entire show a conflict worth caring about. It completely turned what was a tolerable but less than riveting show into some genuinely entertaining TV. Here's hoping when the show comes back this fall the change is permanent.

7) Twin Peaks, "Lonely Souls"

The central question of Twin Peaks — hell, its entire raison d'etre — was finding out who had killed Laura Palmer. Why studio executives thought it was a good idea to force David Lynch to solve this mystery in the middle of the second season; the word is that they were trying to combat Twin Peaks' declining ratings, but what did they think would happen after the mystery was solved? There were plenty of plot threads left to explore in the show, and Kyle MacLachlan's Agent Cooper still had copious amount of weirdness left to explore, but neither the show nor the viewers seemed to care as much any more. This episode was a game-changer, but one that also effectively ended the game.

8) Supernatural, "Lazarus Rising"

Season three of Supernatural ended with Dean in hell — literal, honest-to-Judeo-Christian-God Hell — season four begins with him waking up in a coffin. As the episode progresses, we meet the angel Castiel, who reveals not only that he resurrected Dean, but he did so because God had plans for him. This episode marks the sudden shift of Supernatural from, well, the supernatural, to the divine and demonic, where the Winchester brothers deal with angels, demons, God, Satan, and pretty much everything in-between. It's been a permanent change, and one that has come to define Supernatural.

9) Doctor Who, "The Tenth Planet"

Can you imagine watching Doctor Who in 1966, and seeing the Doctor die? There's no internet gossip, no spoilers, no knowledge of what's coming next. As the Doctor collapses to the floor, his face begins to glow — and slowly, almost imperceptibly a new face begins to appear. Suddenly, instead of William Hartnell, Patrick Troughton jumps up, and a brand-new Doctor is introduced. Can you imagine the shock? Not only has this major change directly influenced Doctor Who storytelling for that point on, it's also the reason why the show has been able to air for most of the last 50 years! Maybe in terms of plot not much changed, but in terms of the franchise, "The Tenth Planet" was an absolutely crucial moment in Doctor Who history.

10) Angel, "Sleep Tight"

Angel was never a particularly upbeat show, but viewers didn't realize how dark the show was going to get until "Sleep Tight" aired in 2002. In it, Wesley — convinced Angel will kill his son according to some prophecy — steals the infant Connor, only to get his throat slit, and someone else takes Connor into a hellish dimension that will result in Connor coming back in just a few episodes as a teenaged demon killer. . Connor's existence defined Angel, both the character and the show, from his birth, and "Sleep Tight" was what defined Connor as a character from that point on. More importantly, this episode split apart the team, creating divides that would never truly heal even by the show's end, forcing the protagonists into conflict with each other at least as much as the bad guys they normally fought. Also, it turned Wesley into the terrifying badass of the later two seasons. Basically, no one escaped this episode of Angel unscathed.

11) Babylon 5, "War Without End"

A scifi show based on prophecy sounds a bit weird, but that's what Babylon 5 was… at least until this episode, in which it was revealed that the Babylon 4 had traveled back in time from the present moment, and that's why Valen had been able to predict the future. Not only does "War Without End" effectively remove all supernatural aspects from Babylon 5, the protagonists — who were able to rely on the prophecies beforehand — were now suddenly on their own, without any more knowledge of the future. They change was subtle but definite, and it affected every character on the show.

12) Dollhouse, "Epitaph One"

Bizarrely, although "Epitaph One" was essential to everything Joss Whedon's Dollhouse was about, it never aired in America. You had to buy the season one DVD in order to watch it, and if you did, it blew your mind. The show, which has previously been about "dolls" imprinted with personalities for various tasks, and the slow awakening of one such doll played by Eliza Dushku, suddenly jumped to 2019, where the world had effectively ended. The memory-imprinting Dollhouse tech has been turned into a weapon, and only a small remnant of humanity is left. The next season becomes a race to prevent this future… one that they do not succeed in.